The present invention relates to equipment utilized in drilling offshore oil and gas wells, and particularly to equipment that is useful in drilling offshore oil and gas wells using a tension leg platform.
In the drilling of offshore wells, it has been the practice to use either fixed platforms or drilling rigs which are fixed in location. The drilling rigs normally are jackup rigs in which long piles are lowered into contact with the ocean floor and jack the drilling platform above the water to provide a fixed platform for drilling operations. As operations moved into deeper waters, it became the practice to use a floating platform which was referred to as a semic-submersible platform that was anchored in place by conventional anchor cables and anchors. In both of these operations, once the wells were drilled and completed, the drilling rig was moved off and replaced by a fixed platform that was supported by means of long columns extending upward from the ocean floor. At times the fixed platform also included drilling equipment so that additional wells could be drilled after the fixed platform was in position.
The use of fixed platforms is limited to roughly 1000 to 1500 feet of water. In deeper waters the cost of the platform and the difficulty in setting the platform becomes prohibitive. In order to solve this problem a platform referred to as a tension leg platform has been developed for use in these extremely depp waters. A tension leg platform refers to a platform in which tension legs are used to draw the platform down below its normal floating level and maintain it in a relatively stable position regardless of wind or wave action.
While the use of a tension leg platform provides a stable drilling platform for use in deep waters, the equipment used in drilling in shallow water cannot alwasy be used in deeper waters. One problem that arises is that the wells must be drilled and cased but cannot be completed due to the lack of space on the platform during drilling operations. Thus, the wellheads must be left on the ocean floor to be subsequently completed after drilling operations have ceased. The temporary abandonment of the wellhead on the seafloor subjects the wellhead to the corrosive action of the seawater as well as the possibility of debris, marine growth and other foreign matter damaging the wellhead or entering the open well casing. Therefore, steps must be taken to protect the temporarily abandoned wellhead until such time as the well is completed and connected to the surface by suitable production tubing.